The infectivity of pandemic 2009 H1N1 and avian influenza viruses for pigs: an assessment by ex vivo respiratory tract organ culture*

BACKGROUND Pigs are thought to act as intermediate hosts in the ecology of influenza viruses of both avian and human origin. The recent development of procedures for pig ex vivo respiratory organ explants has provided new tools for the assessment of influenza virus infection in pigs. OBJECTIVES To use pig ex vivo organ explants to assess the susceptibility of pigs to infection with contemporary viruses, for which there is evidence of human infection and that are thought to pose the greatest threat to pig and human populations. METHODS Pig tracheal, bronchi and lung ex vivo organ explants were infected with both highly pathogenic and low pathogenic avian influenza (AI) virus and the pandemic H1N1 [A(H1N1)pdm/09] virus. Successful infection of explants was detected using a positive-sense RNA real-time RT-PCR assay and anti-nucleoprotein immunohistochemistry. The distribution of cell-surface α2-3- and α2-6-linked sialic acid receptors, the avian- and mammalian influenza A virus-preferred host receptors, respectively, was also characterised for the ex vivo organ cultures and uninfected pig material following necropsy. RESULTS The α2-3 and α2-6 sialic acid receptor staining on tracheal, bronchi and lung organ explant sections showed similar distributions to those seen for pig tissue following necropsy. While the pig ex vivo organ cultures were susceptible to nearly all viruses tested, lower levels of virus were detected in trachea and bronchi after infection. CONCLUSION These results confirm that pigs are susceptible to contemporary viruses that may threaten both veterinary and human health and contribute to the ecology of influenza A viruses.